The present invention generally relates to achieving electrical interfaces between two conductive paths, and more particularly to the construction of sockets for holding the I/O contacts of an IC chip in electrical contact with the compliant tips of an array of conductive probes, typically pogo pin probes. The invention has particular application in contacting ball grid array (BGA) devices wherein electrical contact must be achieved within an array of densely packed solder balls. However, the socket and method of the invention can also be used in connection with IC chips having other contact or lead formations.
Sockets for holding IC chips are well known and widely used in a variety of industrial applications for achieving efficient electrical contact between the I/O contacts of an IC chip and the conductors of a connector, circuit board or the like. Such applications include test sockets for testing IC devices and contactor sockets for removably mounting an IC device to a printed circuit board. The basic function of the socket is to hold the IC chip in a fixed position such that its array of I/O contacts, eg. the solder balls of a BGA, can be contacted by the compliant tips of a corresponding pogo pin array to produce a desired electrical connection. Precise centering of the pogo pins relative to the chip's I/O contacts is critical to this function and generally to the electrical and mechanical performance of the socket. Any misalignment between the pogo pin tips and the I/O contacts can result in a poor electrical interface to the IC chip. Distortion of the I/O contacts can also result when misalignments produce off-center contacts.
In a conventional IC socket, an IC chip is held in a socket base directly over an array of pogo pins. In such socket designs, the chip's I/O contacts are typically centered by referencing the known lateral dimensions of the IC package to the sidewalls of the socket base in which the IC chip is held. The difficulty with this centering approach is that the true position of the I/O contacts within the socket are subject to usual variations that occur in the dimensions of the IC package. The resulting deviations from a true center detrimentally affects the electrical performance of the socket and increases undesirable deformations in the chip's contacts. The problem of contact deformation is particularly acute in BGA devices where deformations in the device's solder balls can detrimentally affect the inspection of the IC device as well as the assembly of the device onto a printed circuit board. Excessive deformation of the extremely small solder balls of micro-BGAs can, for example, cause such IC devices to be rejected during quality control procedures which typically employ laser or other inspection techniques.
Another drawback of conventional IC socket designs is the need to apply a modicum of force to disengage the IC chip from the pogo pins upon removal of the IC chip from the socket. Generally, the tips of the pogo pin probes will, to a certain degree, indent, displace or disturb the chip's I/O contacts, creating a binding force between the chip and pogo pins which must be overcome upon removal. The need therefore exists for an IC socket where the removal of the chip from the pogo pin tips can be facilitated.
The present invention provides an improved IC socket design and method that overcomes the aforementioned drawbacks of conventional IC sockets. The improved IC socket and method of the invention reduces chip positioning errors and more particularly permits extremely accurate centering of the chip's I/O contacts over the tips of the socket's pogo pins. The invention also facilitates the removal of the chip from the IC socket.